When Flu Occurs
The peak of flu season can occur anywhere from late December through March. In the Southern Hemisphere the season is from June to September. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. Seasons are more likely to be severe if the epidemic originates in China. The Centres for Disease Control monitors circulating flu viruses and their related disease activity and provides influenza reports each week from October through May.
The period when an infected person is contagious depends on the age and health of the person. Studies show that most healthy adults may be able to infect others from 2 days prior to becoming sick and for 10 days after they first develop symptoms. Some young children and people with weakened immune systems may be contagious for longer than a week. Infectivity also varies widely between people, with about one in ten being "hyper-infectors" who are responsible for the majority of influenza transmission. While each flu season is unique, it is estimated that, on average, approximately 5% to 20% of people get the flu each year, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 people die on average per year from the complications of flu.
Swine flu is a respiratory illness in pigs caused by a virus. The swine flu virus routinely causes outbreaks in pigs but doesn't usually kill many of them. Swine flu viruses don't usually infect humans, but there have been occasional cases, usually among people who've had direct contact with infected pigs, such as farm workers. Currently medical workers are exploring the possibility of a mutation of known types of the virus, enabling it to pass among species more easily. There have been cases of the virus spreading from human to human, probably in the same way as seasonal flu, through coughing and sneezing by infected people. The virus is an influenza A virus, carrying the designation H1N1. 
In April, 2009, an outbreak of Swine Flu was reported, first in Mexico, and then immediately after other countries as well, giving every indication of a possible pandemic. The Centers for Disease Control and Prevention said the Mexican virus samples match the U.S. virus. The virus is a mix of human virus, bird virus from North America and pig viruses from North America, Europe and Asia. The CDC recommends the use of the flu drugs Tamiflu and Relenza, as there isn't a specific vaccine for the current mutated strain.  It is genetically different from the fully human H1N1 seasonal influenza virus that has been circulating globally for the past few years. The new flu virus contains DNA typical to avian, swine and human viruses, including elements from European and Asian swine viruses.